In one embodiment, this invention relates to a composite seal such as a composite seal for a valve seal ring. In another embodiment, this invention relates to a seal ring such as a seal ring for a valve. In yet another embodiment, this invention relates to a ball valve, such as a trunnion ball valve, having improved sealing capabilities. In a still further embodiment, this invention relates to a method for protecting a seal in a valve from damage by high velocity fluids.
A common seat design of a trunnion mounted ball valve utilizes an O-ring fitted into a groove and retained by a metal snap ring. The exposed portion of the O-ring makes contact with the ball. With the spring force from behind the seat, the O-ring is compressed tightly against the ball and completes the seal. The advantage of the rubber seal is its flexibility. The disadvantage is its low strength. When the sealing pressure approaches 1800 psig the O-ring will be damaged when the ball is cracked open. The rubber O-ring just can not stand the explosive super high speed flow. Another problem encountered when using O-ring seals at high pressure differences is that the O-ring may become squeezed flat, permitting metal to metal contact between the ball and the seat. High torque may be required to break loose the valve, necessitating a larger motor in motor operated valves and possibly creating a safety problem. Another disadvantage is that an extra machining operation is required to provide a groove to accommodate the snap ring required to retain the O-ring.
One way to overcome this problem is to insert a hard, high tensile plastic fitted into a groove in the valve seat. The material can be Nylon (synthetic polymeric amide which has recurring amide groups as an integral part of the main polymer chain), Teflon (polytetrafluroethylene), Delrin (linear polyoxymethylene-type acetal resin), or PEEK (polyetheretherketone) for example. These plastics are 5 to 20 times stronger than most rubbers.
The advantage is that the high speed flow will not cut the plastics. The disadvantage is that the hardness of the plastics makes full sealing contact with the ball difficult when the pressure is low. As a matter of fact, unless the ball is perfectly round, the surface is perfectly smooth, and the spring force very large, the plastic insert will not seal low pressure. This can create safety problems in applications where a low pressure difference may be experienced across the valve, such as in metering applications. The high spring force causes another problem. The torque required to turn the valve at low pressures is higher than the torque required to turn an O-ring sealed valve at low pressures. The higher torque requirement necessitates using a larger motor in motor operated valves, an additional expense. Another disadvantage is that a machining operation is generally required alter the plastic insert is positioned, in order to provide sufficient trueness for sealing.